http://www.fitness.com/exercises/upl...osu-crunch.gif

With both feet flat on the floor, about shoulder width apart, place the small of your back around half-way between the top and the bottom of the BOSU. For this exercise, rest your hands on your thighs.While focusing on your core (abs), roll your body upward and push your hands past, as far as you can, to your knees. Think about pulling with your core, not with your arms or shoulders. Do the exercise slowly to focus on working the core and not using momentum to do each crunch.

More...